I. Field of the Invention
This invention relates generally to electrically controlled devices for making and breaking electrical circuits with a latching means and more particularly to devices for automatically preventing the exhaustion of electrical storage batteries mounted on or installed in the battery.
II. Battery Behavior
In most types of rechargeable batteries, a decrease in energy level is accompanied by a decrease in voltage. A battery also has a minimum energy level below which it loses its storage capacity and power.
When lead-acid batteries, such as those used in automotive electrical systems, are overdischarged, voltage drops sharply and if allowed to remain in this state, changes occur which irreversibly reduce the power capacity of the battery. These changes include lead dropping off the battery plates and the plates sulfating and warping.
A number of common problems cause overdischarging in vehicles. These problems include leaving accessories on, short circuits, corrosion leakage, faulty wiring, and charging system failures.
The increasing complexity of electrical and electronic systems has contributed to the frequency of failures. Lead-acid battery failures from overdischarge are common. The average life of a battery is only abut 3 years. Over 120 million replacement batteries are sold every year. Sadly, failed batteries cause grief, are inconvenient, expensive to replace, and a danger to the environment.
Fortunately, if discharge is stopped at a minimum voltage level, the battery will equalize chemically, increase in voltage, and regain its ability to deliver substantial power. For a 12 volt, lead-acid battery the minimum voltage level is abut 11.0 volts.
Overdischarge and voltage drop in banks of nickel-cadmium batteries in series can completely discharge a single cell and impose a reverse voltage on it, damaging the cell's power capacity.
III. Description of Prior Art A. Latching Circuit Breakers
Previous electrically controlled, latching, circuit breaker-makers U.S. Pat. Nos. 453,572 by Baumann, 1,140,491 by Anderson, 1,485,750 by Wolfe, 1,980,458 by Westerfelhaus, 2,081,618 by Dunn, and 4,628,289 by Huber have employed physically large electromagnetic coils and mechanisms in relation to their current carrying capacity, and relatively complex mechanisms.
B. Battery Mounted Circuit Breakers
U.S. Pat. No. 4,310,817 by McNeil, is an electrically controlled circuit breaker that mounts on a battery post. It is added as a battery accessory without changing the existing electrical wiring. Although the device is adequate to perform this function, its physical dimensions are large so as to interfere with other equipment such as close fitting vehicle hoods and accessory equipment. Other shortcomings are its relatively large actuating current, the need to be manually reset, and its relatively large cost to manufacture.
U.S. Pat. Nos. 4,782,240 by Davidson and 4,798,968 by Deem are circuit breakers mounted on a battery post controlled by means other than a controllable electrical signal. Both require connection of the load to a nut and bolt terminal which most often requires alteration of existing connectors. The Davidson design connects to the battery terminal by way of a metal plate with a hole through it and lacks resistance to vibration. The Deem device breaks circuits when there is an external impact, and although meant to be mounted in battery space, it is so large as robe interfered with by the position of ventilation caps found on some common battery designs and by accessories above the battery such as tight fitting compartment hoods.
C. Automatically Controlled Circuit Breakers
A number of patents use automatically controlled circuit breakers to prevent overdischarge of a battery.
A common shortcoming of these designs is they are not easily transferred from one electrical system to another. Installing them to protect an existing electrical system requires alteration of wiring and ample space for physical mounting. This makes it expensive and awkward to add them to battery systems such as vehicle electrical systems.
U.S. Pat. No. 4,149,093 by D'Alessio uses a complex control circuit and a special circuit breaking mechanism to switch a vehicle battery. Shortcoming of the device include the manufacture of a relatively expensive device and the inconvenience of manually resetting the mechanism.
One shortcoming of the remaining designs for use in vehicles, is they do not control power to the starter motor. If an electrical leak or short circuit occurs in this branch circuit, overdischarge can damage the battery.
U.S. Pat. No. 4,902,956 by Sloan uses a complex circuit with a microprocessor and a conventional solenoid to control accessory circuits. Its shortcomings include the manufacture of a relatively expensive device and the inconvenience of manually resetting a mechanism.
The remaining prior art cited of automatically controlled circuit breakers are electrical and electronic circuit designs. They monitor battery voltage and use a voltage sensitive means to open a connecting relay when voltage drops below a certain value, breaking the circuit.
In these designs the circuit breaker is reset by either manually manipulating a switch, or automatically with a relay energized by a voltage sensitive circuit. The manually resetting designs are either inconvenient to reset or require extra wiring. The automatically resetting designs have the shortcoming of cycling on and off after they break the circuit. As an example, consider the case of an automobile when the headlights are left on. The battery discharges to its shut-off voltage level and the circuit breaker opens, interrupting power to the headlights. With no load on the battery, battery voltage spontaneously increases to an upper turn-on voltage level and the circuit breaker recloses, supplying electricity to the headlights or other drains. Once more the battery discharges to its lower shut-off voltage and this on-off cycling repeats. This causes further deterioration of the battery until the load is removed or the battery recharged.
Another shortcoming of the following designs is that they use relays that are not self-latching and require holding current to keep relay contacts closed during normal operation. This is a costly energy drain on the power source, as it drains the battery, especially over long periods of time.
U.S. Pat. Nos. 2,051,514 by Block, 2,485,727 by Gallina, 3,395,288 by Von Brimer, and 3,623,131 by Russel use voltage sensitive relay switches to control electric power.
A number of patent designs replace the voltage sensitive relay with sensing elements and a less expensive conventional relay to save costs and improve switching voltage accuracy.
U.S. Pat. Nos. 3,474,296 by Rickey, and 3,395,288 by Vone Brimer use a zener diode and a relay to replace the voltage sensitive relay.
U.S. Pat. Nos. 3,646,354 by Von Brimer, and 4,005,344 by Gaind use a transistor and relay to control the circuit.
U.S. Pat. Nos. 3,648,145 by Meyer, and 2,959,717 by Conger use a zener diode, transistor and relay to control the circuit.
D. Similar Electronic Designs
U.S. Pat. No. 3,321,754 by Grimm uses zener diodes, transistors, and relays to provide undervoltage and overvoltage indicators. The main shortcoming to our application is that it performs no circuit breaking function.